A Review of Criticality Accidents A Review of Criticality Accidents

21. Novosibirsk Chemical Concentration Plant, 15 May 1997
Uranium oxide slurry and crust, U(70), in the lower regions of two parallel vessels; multiple excursions;
insignificant exposures.
This accident occurred in Building 17 where highly
enriched U(90) fuel rods were fabricated from UO 2
and aluminum, using the powder metallurgy method.
The final process prior to placing a rod into its aluminum
cladding was chemical etching. The objective of
this operation was to remove microscopic defects on
the rod surfaces to ensure tight contact between the rod
and its cladding. The major components of the process
equipment are shown in Figure 32.
The etching process involved consecutively immersing
a batch of rods into three separate vessels filled
with sodium hydroxide (alkali), water, and nitric acid,
respectively. As a result of a chemical reaction with the
sodium hydroxide, some of the aluminum particles on
the surface of the rods formed the precipitates, NaAlO 3
and Al(OH) 3 . The etching process leached a small
fraction of the uranium dioxide from the rods which
was also deposited on the bottom of the alkali vessel.
The rods were then washed in the second vessel with
water to remove as much alkali as possible. In the final
vessel of nitric acid, any traces of alkali were neutralized
and the rods were etched to their final size by
dissolving UO 2 particles from their surfaces.
50
Alkali Water Acid
59-A
Receiver
Vessels
59-B
Collection
Vessel
To Recovery
Figure 32. Major components of the chemical-etching
process.
Once the etching process with one batch of rods
was completed, the contents from all three immersion
vessels was gravity drained to a 130 mm diameter
cylindrical collection vessel. As the collection vessel
filled, the etching solution was pumped to receiver
vessels 59-A and 59-B through a 64 mm diameter pipe
that was about 100 m in length. Eventually, the etching
solution would be transferred from the receiver vessels
to the uranium recovery section of the building.
The etching process had been standardized 13 years
before the criticality accident. During this 13 year
period, the controlled parameters were the number of
rods per batch, the temperature, the concentration of
the reagents, and the duration of the immersion
operations. Uranium content in the etching solutions
was not measured. In fact, since all of the equipment,
with the exception of the immersion vessels, was of
favorable geometry (according to design and records),
there was no capability within the building to determine
the uranium content of the etching solutions.
Precipitate formation and uranium deposition in the
service piping was also not monitored.
In 1996 a solid UO 2 deposit was discovered in the
collection vessel when it was opened and inspected.
The 5.5 kg deposit (uranium mass fraction of ~69%)
was gradually removed by dissolving it with nitric
acid. Analyses showed that the deposit had been
forming for over 10 years since it contained some
uranium enriched to only 36% by weight. Uranium
enriched to 36% by weight had not been processed
since around 1986 when a switch to U(90) was made.
Despite the discovery of this deposit, the logical
search for similar deposits in the service piping and in
the receiver vessels was not initiated, primarily
because the criticality safety limits for those components
did not include requirements to monitor for
uranium accumulation. In fact, for the purposes of the
annual material inventory, the entire building was
designated as one fissile material balance area.
Therefore, the small fraction of uranium being deposited
in the receiver vessels per batch of rods went
unnoticed because of the large size of the fissile
material balance area.
The receiver vessels were of slab geometry, with
common inlet and outlet lines for the transfer of
solutions. Each vessel was 3.5 m in height and 2 m in
length, with a design thickness of 100 mm and an
operating volume of ~650 l. Both vessels were made
from 4 mm thick stainless steel. The distance between
the vessels was 0.8 m, and they were mounted ~0.75 m
above the concrete floor. Dimensional control of the
thickness was provided by internal steel rods welded